IOS 15 iPhone13

The grey scan text button is quite small and appears at the bottom right of the pic (its quite hard to spot it sometimes).

The picture you're scanning text from needs to be detected as having text in it - I'm in the UK and had pics with text picked up by it whilst others aren't - not sure why that would be other than maybe image quality.

I did have it pick some text from a photo of a hand written note but it ignored parts which were clearly readable.

I've enlarged and circled what you should be looking for.

When you tap it it turns blue and the detected text is highlighted. This is in the photo app

In the camera app it works similarly, but the icon turns yellow

This is what Apple provides in their documentation about Live Text.

Available on iPhone with A12 Bionic and later. Live Text is currently supported in English, Chinese, French, Italian, German, Portuguese, and Spanish.

Live text has to be turned on in Settings>Camera.

Ahh... I thought you were talking about the OCR function from photos.

It doesn't look like Visual Search is active here either.